1. Field of the Invention
This invention relates to an aqueous ink composition. More particularly, this invention relates to an aqueous ink composition for use in writing instruments including an ink passage made of a plastic material.
2. Description of the Prior Art
There are many examples of writing instruments in which an ink passage is made of a plastic material. Plastic molded articles are frequently used as nibs, feed elements or ink reservoirs in marking pens, as feed elements or ink reservoirs in fountain pens, and as tips, feed elements or ink reservoirs in ball-point pens for aqueous inks.
Most plastic materials intrinsically have low affinity for water, and are difficult to wet with aqueous inks. This frequently causes an interruption of ink flow in ink passages made of a plastic material. If a part which requires the presence of an extremely narrow ink flow channel, such as a nib or a feed element, is made of a plastic material, the channel width cannot be made narrow to the same extent as in a fibrous bundle, and, therefore, the capillary action in such a channel is weaker than in a fibrous bundle, thus, the tendency toward an interruption in ink flow increases. Accordingly, an "ink swallowing phenomenon" tends to occur in a writing instrument including such a plastic part whereby ink drops into the reservoir when the writing instrument has been left with the nib turned upside-down or when the instrument has been subjected to impact. As a result, the ability to write instantaneously after the occurrence of such a phenomenon becomes impossible, or inking failure tends to occur during writing. In a fountain pen including an ink cartridge made of a plastic material such as polyethylene, when the ink has been partly consumed to form a space within the cartridge, the wetting of the inner wall of the cartridge with the aqueous ink becomes poor. Hence, there is a tendency for the ink not to flow in the direction of the nib, but to remain within the cartridge.
Heretofore, the following techniques have been employed to use an aqueous ink in a writing instrument containing a part made of a plastic material.
(1) A surface active agent, as a wetting agent, is incorporated into the plastic material itself.
(2) A surface active agent is incorporated into the ink to increase the affinity of the ink for the plastic material.
(3) The surface of the plastic material is roughened by treatment chemically or with ozone.
When technique (1) above is used, the surface active agent is frequently extracted into the ink when ink is contacted with the plastic material for long periods of time. Thus, blotting occurs frequently with such an ink.
If a surface active agent is added in an amount sufficient to wet the plastic material as in technique (2) above, the writing tends to blot on the paper to a great extent.
In technique (3) above, roughening the surface of the plastic portion of the writing instrument involves difficulties. Treatment of a fine channel in the nib or feed element is time-consuming, and complete treatment is extremely difficult to accomplish.